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The Millennium GalaxyCatalogue: morphological classification and bimodality in the colour-concentration plane . (astro-ph/0602240). S.P.Driver et al. The Millennium Galaxy Catalogue. 37.5 deg 2 region of sky. 2.5m Isaac Newton Telescope in La Palma.

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The Millennium GalaxyCatalogue:

morphological classification and bimodality in the colour-concentration plane


S.P.Driver et al.

The Millennium Galaxy Catalogue

37.5 deg2 region of sky

2.5m Isaac Newton Telescope in La Palma

The MGC lies within the Two Degree Field Galaxy Redshift

Survey (2dFGRS) Northern Galactic Cap region and the Sloan Digital Sky (SDSS) Early Data Release Region.

The MGC-bright imaging catalogue contains 10 095 galaxies

(13.0 < BMGC < 20.0)

for which redshifts have now been obtained for 9 696 resulting

in a global completeness of 96 per cent

BMGC < 19.0 mag. In these cases the sample size is reduced to 3 492 galaxies for which redshifts have been obtained for 3 487 indicating a global completeness of 99.9 per cent.

Multiple evolutionary paths, mechanisms and time-scales do exist. From a theoretical perspective there are several proposed modes of evolution:

  • monolithic collapse (Eggen,Lyndon-Bell& Sandage 1962; Sandage 1990),

  • hierarchical merging (White &Rees 1978; Fall & Efstathiou 1980),

  • gas infall (Blumenthal et al. 1986)

  • satellite accretion (Searle & Zinn 1978),

  • secular evolution (see review by Kormendy & Kennicutt 2004)

  • splashback (Fukugita & Peebles 2005).

The number of secondary (and mainly environmentally dependent) processes is even higher

  • tidal formation (Barnes & Hernquist 1992),

  • ram-pressure stripping (Gunn & Gott 1972),

  • Strangulation (Balogh,Navarro & Morris 2000)

  • harassment (Moore et al. 1996),

  • Squelching (Tully et al. 2002),

  • threshing (Bekki, Cough & Drinkwater 2001),

  • cannibalism(Ostriker & Hausman 1977)

What is the fundamental way to divide the galaxy population if at all?

Which plane is more fundamental and what these bimodalities might be telling us.

Our argument, which we develop throughout this paper, is that they all reflect the two component nature of galaxies (as opposed to two distinct classes of galaxies) and that the fundamental division is between the disc and bulge components, each of which may have a distinct formation mechanism acting over two distinct eras.

The MGC morphologies if at all?

E/S0: Smooth, highly concentrated symmetrical systems with

no obvious spiral-arm disc component.

Sabc: Clear two-component bulge-disc system, generally smooth and symmetrical.

Sd/Irr/Pec: Disc only (i.e., no obvious single core),

asymmetric, highly disturbed or multiple-core systems.

The 2dFGRS  parameter

SDSS global and core colours if at all?

Stellar mass-to-light ratios

The MGC Sersic indices

The MGC central surface brightnesses

The MGC half-light radii

We find that continuum fitting of the broad-band colours leads to a good

separation of the two populations which we label ``old'' and ``young''

(and corresponds to the ``red'' and ``blue'' peaks respectively).

The E/S0 systems lie predominately in the old, red peak and the Sd/Irrs

lie wholly in the young, blue peak with the Sabc class straddling both

peaks and the divide. We take this as strong evidence that the galaxy

population does not consist of two classes but two components, consistent with the classical idea of bulges and discs.

The red peak hence constitutes older bulges and the blue peak younger discs. We advocate that this joint-bimodality (i.e., colour and concentration), may reflect two distinct formation mechanisms occurring over different time-scales.

  • Luminosity distribution summary leads to a good

  • Morphological classification based on the eye, including Artificial Neural Networks which use a training set, are fraught with difficulty because of the subjective nature of the classification process.

  • Galaxies can also be effectively separated into two populations using structural division in log(n) space. The populations constitute a concentrated and diffuse population.

  • Colour appears to segregate the galaxy population more effectively than any other single measurement (i.e., Sersic index or surface brightness).

  • Both global and core (u-r)_{g,c} colours segregate the population well into blue and predominantly luminous red systems.

  • The core colour appears to better segregate the two populations (in M-alpha space) than the global colour, probably due to the blending of colours from red bulges embedded in blue discs.